Seminar: Storm Boris (2024) in present and future climate

Part of the Cycle of seminars: Understanding and managing climate risk

Storm Boris (2024) in present and future climate: a dynamics-based contextualization and meta-attribution

The response of mean and extreme precipitation to anthropogenic global warming (AGW) stems both from warming of the troposphere and dynamical changes in the large-scale circulation, especially in vertical atmospheric motions. These two effects may compensate each other, leading to substantial uncertainties in future projections of extreme precipitation events. The uncertainty in the expected response, as well as the more complex nature of precipitation extremes with respect to temperature extremes, further complicates climate attribution attempts for this type of events. Starting from the case study of Storm Boris, that led to major floods over central Europe in mid-September 2024, we disentangle the thermodynamical and dynamical contributions to this event adopting a dynamics-centered, analog-based approach. The first step is the observation of some key dynamical features, whose representation is deemed crucial to obtain realistic analogs of the rainfall event: those features are the presence of a slow-moving, upper-level potential vorticity (PV) cutoff, the peculiar track of the surface cyclone associated with Storm Boris, and the presence of anomalously strong forcing for ascent. Following this dynamical guidance, we scan a large ensemble of 1000-year present-day and end-of the-century simulations from the Community Earth System Model v1 (CESM1) to identify several circulation analogs of Storm Boris. We find that the combined use of upper-level PV and of a surface cyclone identification algorithm substantially improves the quality of the analogs, especially for what concerns the magnitude and the location of the associated precipitation. Interestingly, we notice that the response of Boris-like events to AGW depends on the period used to determine the analogs: analogs of Storm Boris restricted to the same season of occurrence (i.e., August to October) feature on average less precipitation in a warmer climate, due to an overall weakening of upper-level-driven ascent over Europe. However, analogs of Boris determined over the full year feature a slightly higher precipitation in a warmer climate, pointing towards a seasonality shift of the events. The results obtained from the analog-based approach are then compared with an unconditional, statistics-based approach focusing only on the seasonal and yearly maxima of precipitation. This second approach allows to recover the expected intensification of extreme precipitation in a warmer climate – at the price, however, of considering events that do not necessarily have the same large-scale circulation pattern as Storm Boris. To conclude, we discuss some implications of our results for statistics-based and analogs-based climate attribution approaches.

Contact: Marco Gaetani — marco.gaetani@iusspavia.it